Script Loader: Check if $_wp_admin_css_colors is set in wp_style_loader_src().

This aims to avoid PHP warnings if the `colors` dependency is loaded or the `style_loader_src` filter is used in a context where the `$_wp_admin_css_colors` global does not exist.

Follow-up to [7976].

Props crstauf, petitphp, SergeyBiryukov.
Fixes #61302.

git-svn-id: https://develop.svn.wordpress.org/trunk@61388 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/script-loader.php

@@ -2090,8 +2090,8 @@ function wp_style_loader_src( $src, $handle ) {
 			$color = 'fresh';
 		}
 
-		$color = $_wp_admin_css_colors[ $color ];
-		$url   = $color->url;
+		$color = $_wp_admin_css_colors[ $color ] ?? null;
+		$url   = $color->url ?? '';
 
 		if ( ! $url ) {
 			return false;

tests/phpunit/tests/dependencies/wpStyleLoaderSrc.php

@@ -0,0 +1,26 @@
+<?php
+
+/**
+ * Test wp_style_loader_src().
+ *
+ * @group dependencies
+ * @group scripts
+ *
+ * @covers ::wp_style_loader_src
+ */
+class Tests_Dependencies_wpStyleLoaderSrc extends WP_UnitTestCase {
+
+	/**
+	 * Tests that PHP warnings are not thrown when wp_style_loader_src() is called
+	 * before the `$_wp_admin_css_colors` global is set.
+	 *
+	 * The warnings that we should not see:
+	 * `Warning: Trying to access array offset on null`.
+	 * `Warning: Attempt to read property "url" on null`.
+	 *
+	 * @ticket 61302
+	 */
+	public function test_without_wp_admin_css_colors_global() {
+		$this->assertFalse( wp_style_loader_src( '', 'colors' ) );
+	}
+}